Police in Swaziland have arrested about 50 people ahead of protests against sub-Saharan Africa's last absolute monarchy, activists say.

Most of those detained were later freed and hundreds of people marched through Manzini, Swaziland's commercial centre.

Several South Africans trade unionists were prevented from taking part in the march and deported.

King Mswati III and his 13 wives are accused of living in luxury while most Swazis are poor.

The king is also criticised for having so many partners in a country with one of the world's highest rates of HIV infection - 26% of those aged 15-49.

Human rights group Amnesty International has condemned the arrests.

"The arbitrary arrest of these political activists, lawyers, trade unionists and journalists is nothing short of police harassment and intimidation," the group said in a statement.

Police spokeswoman Wendy Hleta said the foreigners were arrested because "we felt that they had no right to interfere" in Swazi affairs, reports the AP news agency.

A spokesman for the South Africa trade union federation, Cosatu, said two of those arrested had not been freed and their whereabouts were not known, according to the Reuters news agency.

Officials from the banned Peoples United Democratic Movement of Swaziland (Pudemo) were also reportedly detained, including its leader Mario Masuku.

The US-funded pro-democracy group, the Open Society Initiative for Southern Africa (OSISA), says two of its members were among those detained.

"It is time that the world understood what kind of regime runs Swaziland. It is a regime that has no respect for human rights, no respect for the rule of law and no respect for democracy," said OSISA head Sisonke Msimang.